What you would like as solution? Isn't it a de-facto-standard that dark means "selected"?

In J3 we have for example this,which for me is more confusing.

As you can see in the animated radio switcher: dark is NOT the selected option!

A simple solution:
Set the labels inside .btn-group.radio to opacity: .5;, and set the active labels back to opacity: 1;.

The blue color on the selected option instead of gray would be better as well, as of the original post by Peter

Please submit a pr

Seeing this would need a change in the css, I am at a loss.
There is no media folder in the repo and no idea where the template.scss grabs the necessary styles from or where to change the source btn-group styling.

I guess you need to find a real developer to help you use composer, npm and git :)

I guess so. So my job is done here. I reported the issue. If someone wants to fix it, fine. If not, ??‍♂️

For reference:
The source files for the media files are usually here: https://github.com/joomla/joomla-cms/tree/4.0-dev/build/media_source
However for templates, they are in the respective template folder (https://github.com/joomla/joomla-cms/tree/4.0-dev/templates/cassiopeia/scss and https://github.com/joomla/joomla-cms/tree/4.0-dev/administrator/templates/atum/scss
This is done so they end up in the distributed CMS and people can reuse them for their own templates.

Thanks, I got that for.
This whole styling is too baked in to bootstrap. So I am dropping this.